Optical reticle substrate inspection apparatus and beam scanning method of the same

ABSTRACT

An optical reticle substrate inspection apparatus is provided with a laser, a first acoustooptical element which scans a laser beam output from the laser, an a second acoustooptical element which generates a virtual image with a concave lens effect to the laser beam output from the first acoustooptical element. The optical reticle substrate inspection apparatus is further provided with a concave lens arranged on the output side of the laser beam of the second acoustooptical element and an optical system which images the virtual image on a reticle substrate being an object to be inspected. The concave lens magnifies the laser beam in a perpendicular direction to the scanning direction by the first acoustooptical element.

CROSS REFERENCE TO RELATED APPLICATIONS

This a divisional of U.S. patent application Ser. No. 09/998,521, filedNov. 29, 2001, in the name of Motonari Tateno now U.S. Pat. No.6,538,795.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical reticle substrate inspectionapparatus that inspects a defect and a line width of a reticle substrateand a beam scanning method thereof, particularly, to an optical reticlesubstrate inspection apparatus for improving stability when a laserlight source having a short wavelength is used and a beam scanningmethod thereof.

2. Description of the Related Art

In the optical reticle substrate inspection apparatus that inspects adefect and a line width of a reticle substrate, there exists anapparatus in which detection light is converged onto a substrate todetect the defect and the line width by transmission light or reflectionlight. In such a apparatus, a spot of a laser beam (a beam spot) isscanned in one direction parallel with a surface of the reticlesubstrate (hereinafter, this direction is referred to as a Y-axisdirection), and a stage on which the reticle substrate is mounted ismoved in an isokinetic manner in a direction parallel with the surfaceof the reticle substrate and a perpendicular direction to the Y-axisdirection (hereinafter, this direction is referred to as an X-axisdirection), and thus inspecting the defect and the like of the reticlesubstrate. As a scanning method of the beam spot, a method in which thebeam spot is optically scanned is generally used in order to processinspection in a short time, and the beam scanning method using anacoustooptical element has been conventionally adopted.

In such a beam scanning method, aberration occurs to an angle of thescanning beam when the beam is converged by a normal cylindrical lens,and defocus occurs on the reticle substrate surface. For this reason, amethod is adopted where the beam is converged using a lens effect by theacoustooptical element to reduce the defocus. A method converging thebeam using the two acoustooptical elements is described in The U.S. Pat.No. 3,851,951, for example. And a substrate inspection apparatus thatadopted the method is described in Japanese Patent Laid-Open(unexamined) No. Hei 6-294750. FIG. 1 is a schematic view showing theconventional optical reticle substrate inspection apparatus.

In the optical reticle substrate inspection apparatus, a laser lightsource 1, an acoustooptical element 2 and a group of cylindrical lenses3 are arranged in this order in a rectilinear direction of a laser beamoutput from the laser light source 1. The acoustooptical element 2 isarranged so as to scan the laser beam output from the laser light source1 in a perpendicular direction to the straight line by frequencymodulation, and the group of cylindrical lenses 3 is arranged so as tomagnify the laser beam output from the acoustooptical element 2 only inthe scanning direction by the acoustooptical element 2.

An acoustooptical element 4 a which gives a cylindrical lens effect tothe laser beam output from the group of cylindrical lenses 3 is furtherprovided to the optical reticle substrate inspection apparatus. Atransducer 19 which oscillates ultrasonic to the acoustooptical element4 a is attached to one end portion of the acoustooptical element 4 a.The acoustooptical element 4 a is arranged such that a side where thetransducer 19 is attached, that is, a side where the ultrasonic isinput, is closer to the group of cylindrical lenses 3. Furthermore, agroup of the cylindrical convex lenses 18 that converges the laser beamoutput from the acoustooptical element 4 a is arranged in the opticalreticle substrate inspection apparatus. Moreover, a relay lens 24 whichpropagates the laser beam passed through the convergence spot to anoptical system (not shown) in a post-step is arranged on a positionapart from the convergence spot formed by the group of cylindricalconvex lenses 18. In addition, an objective lens (not shown) is arrangedbetween the optical system and the reticle substrate being an object tobe inspected, and a detector (not shown) that detects intensity and thelike of the laser beam passed through the reticle substrate is furtherprovided.

In the optical reticle substrate inspection apparatus constituted inthis manner, scanning is performed in an angle made by an incidentdirection and an output direction to the laser beam output from thelaser light source 1 by utilizing the frequency modulation by the firstacoustooptical element 2. An optical path of the laser beam transitsfrom a path 12 to a path 13 due to this process. When the laser beam hasits optical path on the path 12, the laser beam is magnified by thegroup of cylindrical lenses 3 and output as a laser beam 30 with a widthin the scanning direction.

Further, transducer 19 outputs a series of ultrasonic which is sweptsuch that a wavelength lengthens in linear state as the passage of time,that is, the wavelength in a forefront portion 21 becomes shorter thanthat of an aftermost portion 20. A plurality of parallel lines betweenan aftermost portion 20 and a forefront portion 21 in FIG. 1 show thatthe wider the distance between the lines the longer the wavelength. Thesecond acoustooptical element 4 a, when the laser beam 30 is madeincident thereto, outputs the laser beam 30 while converging it withfunctioning as a cylindrical convex lens by the ultrasonic oscillatedfrom the transducer 19. The laser beam 30 output from the acoustoopticalelement 4 a is further converged by the group of cylindrical convexlenses 18 to form a convergence spot 22. When the laser beam has itsoptical path on the path 13, a convergence spot 23 is formed on aposition off from the convergence spot 22 in the scanning direction. Thelaser beam 30 passed through the convergence spot 22 is made incident tothe relay lens 24 while magnifying its width again. Then, theconvergence spot 22 is imaged on the reticle substrate via the opticalsystem and the objective lens, and the detector detects the intensityand the like of the transmission light.

Note that the same detection can be performed even if the acoustoopticalelement 4 a is arranged such that the side where the ultrasonic is inputis made far from the group of cylindrical lenses 3 and the ultrasonic isswept such that the wavelength becomes shorter in linear state as thepassage of time.

However, in recent years, although higher resolving power has beendemanded for an apparatus for inspecting the reticle substrate withdemand for finer pattern, there is a problem that the conventionaloptical reticle substrate inspection apparatus cannot provide sufficienthigh resolving power.

As a method of obtaining the high resolving power, a method isconsidered where the convergence spot size on the reticle substratesurface is made small by shortening the wavelength of the detectionlight. Progress of shorter wavelength has been made also in the field ofexposure due to advancement in technology. It is important that thewavelength of the detection light is shortened to make it close to thatof an exposure light because the way how the defect appears changesdepending on a detection wavelength. However, in the case of shorteningthe wavelength of the detection light, a usable material of theacoustooptical element is limited due to a problem of materialabsorption regarding the scanning method where convergence is performedby the conventional acoustooptical element. For example, since telluriumdioxide, which has been generally used as the acoustooptical element,does not transmit the detection light having the wavelength of 300 nm orless, the acoustooptical element made of quartz, which has a sonic speedof 5960 m/sec being about ten times faster than comparing to telluriumdioxide, needs to be used. However, there is a problem that a focallength lengthens when the acoustooptical element made of quartz is used,because a focal length of a cylindrical lens effect by theacoustooptical element is inversely proportional to the wavelength oflight output from the light source and proportional to a square of thesonic speed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide the optical reticlesubstrate inspection apparatus that can improve scanning stability whenthe laser light source having a short wavelength is used and the beamscanning method thereof.

An optical reticle substrate inspection apparatus according to thepresent invention comprises a laser, a first acoustooptical elementwhich scans a laser beam output from said laser, a second acoustoopticalelement which generates a virtual image with a concave lens effect tosaid laser beam output from said first acoustooptical element, a concavelens arranged on the output side of said laser beam of said secondacoustooptical element, and an optical system which images said virtualimage on a reticle substrate being an object to be inspected. Theconcave lens magnifies said laser beam in a perpendicular direction tothe scanning direction by said first acoustooptical element.

In the present invention, the first acoustooptical element performslaser beam scanning. Further, the second acoustooptical elementgenerates a virtual image whose aberration is reduced to the laser beam,and the concave lens adjusts the shape of the virtual image in a circle,for example. Then, the optical system images the virtual image on thereticle substrate. As described, in the present invention, since not theconvergence spot but the virtual image is imaged on the reticlesubstrate and its scanning is performed, stable scanning can beperformed even if the laser beam having a short wavelength is used.

A beam scanning method of an optical reticle substrate inspectionapparatus according to the present invention comprises the steps ofgenerating a virtual image with an acoustooptical element to a laserbeam, and imaging said virtual image on a reticle substrate being anobject to be inspected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a conventional optical reticlesubstrate inspection apparatus.

FIG. 2 is a schematic view showing a structure of an optical reticlesubstrate inspection apparatus according to a first embodiment of thepresent invention.

FIG. 3 is a schematic view showing a beam scanning method of the opticalreticle substrate inspection apparatus according to the first embodimentof the present invention.

FIG. 4 is a schematic view showing a structure of an optical reticlesubstrate inspection apparatus according to a second embodiment of thepresent invention and its beam scanning method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be specificallydescribed with reference to the accompanying drawings as follows. FIG. 2is a schematic view showing a structure of an optical reticle substrateinspection apparatus according to a first embodiment of the presentinvention.

In the optical reticle substrate inspection apparatus according to thefirst embodiment, similarly to the conventional apparatus, the laserlight source 1, the acoustooptical element 2 and the group ofcylindrical lenses 3 are arranged in this order in the rectilineardirection of the laser beam output from the laser light source 1. Alaser wavelength of the laser light source 1 is 300 nm or less, forexample. The acoustooptical element 2 is made of quartz, for example,and is arranged so as to scan the laser beam output from the laser lightsource 1 in a vertical direction to the straight line by the frequencymodulation. The group of cylindrical lenses 3 is arranged so as tomagnify the laser beam output form the acoustooptical element 2 only inthe scanning direction by the acoustooptical element 2. Note that thescanning direction by the acoustooptical element 2 corresponds to theY-axis direction.

In this embodiment, an acoustooptical element 4 showing the lens effectis further provided for the laser beam output from the group ofcylindrical lenses 3. The transducer 19 is attached to one end portionof the acoustooptical element 4. The acoustooptical element 4 isarranged such that a side where the transducer 19 is attached, that is,a side where the ultrasonic is input, is closer to the group ofcylindrical lenses 3. Furthermore, in this embodiment, a group ofcylindrical concave lenses 5, which magnifies the laser beam output formthe acoustooptical element 4 only in a perpendicular direction to thescanning direction and outputs it, is provided. A numerical aperture ofthe group of cylindrical concave lenses 5 equals a numerical aperture ofthe acoustooptical element 4 when it functions as the cylindricalconcave lens. Therefore, the shape of the virtual image formed by theacoustooptical element 4 and the group of cylindrical concave lenses 5is the circle.

Moreover, an optical system 8 is provided which images the laser beamoutput from the group of cylindrical concave lenses 5. A relay lens 7which propagates the laser beam output from the group of cylindricalconcave lenses 5 to the optical system 8 in the post-step is arrangedbetween the group of cylindrical concave lenses 5 and the optical system8. The optical system 8 has a structure same as that of a conventionalsystem. For example, the optical system is provided with a group oflenses necessary for imaging, a measure for detecting fluctuation of theintensity and the like of the laser beam, a splitter necessary in amulti-beam inspection method, a detector for detecting a reticlesubstrate reflection light and the like. Further, an objective lens 9that performs convergence close to a theoretical spot limit is arrangedbetween the optical system 8 and a reticle substrate 10 being an objectto be inspected. Furthermore, a detector 11 that detects the intensityand the like of the laser beam passed through the reticle substrate 10is provided to the optical reticle substrate inspection apparatus. Theacoustooptical element 2, the group of cylindrical lenses 3, theacoustooptical element 4 and the group of cylindrical concave lenses 5may constitute a concave lens type scanning portion 6.

Note that a magnification of the group of cylindrical lenses 3 may beset so as to equal a distance between the forefront portion and theaftermost portion when a series of ultrasonic oscillated from thetransducer 19 propagates the acoustooptical element 4, for example.

Next, description will be made for the beam scanning method of theoptical reticle substrate inspection apparatus according to the firstembodiment constituted as above. FIG. 3 is a schematic view showing abeam scanning method of the optical reticle substrate inspectionapparatus according to the first embodiment of the present invention.FIG. 3 shows the laser light source 1 to the relay lens 7.

In this beam scanning method, scanning is performed in the angle made bythe incident direction and the output direction to the laser beam outputfrom the laser light source 1 by utilizing the frequency modulation bythe acoustooptical element 2. At this point, a frequency band modulatedby the acoustooptical element 2 for scanning in the angle can bearbitrarily decided based on a size of the virtual image formed by theacoustooptical element 4, a moving distance of the virtual image withscanning, and a spot size imaged on a surface of the reticle substrate10. The optical path of the laser beam transits from the path 12 to thepath 13 due to this scanning. When the laser beam has its optical pathon the path 12, the laser beam is magnified by the group of cylindricallenses 3 and output as the laser beam 30 with the width in the scanningdirection.

Further, the transducer 19 outputs a series of ultrasonic which is sweptsuch that the wavelength becomes shorter in linear state as the passageof time. A plurality of the parallel lines between an aftermost portion25 and a forefront portion 26 in FIG. 3 show that the wider the distancebetween the lines the longer the wavelength. A distance between theforefront portion 26 and the aftermost portion 25 of a series of theultrasonic equals the width of the laser beam 30. When the laser beam 30is made incident to the acoustooptical element 4, diffraction of thelaser beam 30 occurs by an ultrasonic pulse, in which the frequency isswept such that the wavelength becomes shorter in linear state from theforefront portion 26 to the aftermost portion 25 as described above. Atthis time, since the wavelength of the ultrasonic is different inaccordance with a spatial position of the laser beam 30, Bragg angle isalso different, and thus the concave lens effect occurs in thisembodiment. Accordingly, the acoustooptical element 4 functions as theconcave lens, and a circular virtual image 14 is formed on a side wherethe laser beam 30 is made incident by a combinational function with thegroup of cylindrical concave lenses 5. Further, when the laser beam hasits optical path on the path 13, a circular virtual image 15 is formedat a position off from the virtual image 14 in the scanning direction.

The laser beam 30 output from the acoustooptical element 4 is madeincident to the optical system 8 via the group of cylindrical concavelenses 5 and the relay lens 7 while magnifying its width. Then, theoptical system 8 controls a traveling direction, an aperture and thelike of the laser beam 30, and makes the virtual image 14 to be imagedas a spot 17 on the surface of the reticle substrate 10 via theobjective lens 9. The spot 17 imaged on the surface of the reticlesubstrate 10 moves in the Y-axis direction with an angle scanning by theacoustooptical element 2, and when the laser beam output from theacoustooptical element 2 has its optical path on the path 13, thevirtual image 15 is imaged on the surface of the reticle substrate 10 asa spot 16. Then, the intensity and the like of the transmission lightare detected by the detector 11.

In such a scanning method, assuming that a propagation speed ofultrasonic in the acoustooptical element 4 is V (m/sec.), the wavelengthof the laser beam is (m), a sweeping time of ultrasonic (output time ofa series of ultrasonic) in the transducer 19 is T (sec.), and the bandof ultrasonic oscillated from the transducer 19 is f (Hz), a focallength f (m) when the acoustooptical element 4 shows the cylindricalconcave lens effect is expressed by the following expression 1.$\begin{matrix}{f = {\frac{V^{2}T}{\lambda \quad \Delta \quad f}}} & \left\lbrack {{Expression}\quad 1} \right\rbrack\end{matrix}$

This expression 1 also shows that the focal length is inverselyproportional to the wavelength of the incident laser and proportional toa square of the ultrasonic speed. Thus it is understood that the focallength lengthens when the acoustooptical element made of quartz is used,which has a fast ultrasonic propagation speed of 5960 (m/sec.),comparing to the acoustooptical element made of tellurium dioxide havingthe ultrasonic propagation speed of 620 (m/sec.) as in the foregoing.

Since this embodiment has a constitution where the acoustoopticalelement 4 is used so as to generate the concave lens effect and relaythe virtual image, a total optical path length can be shortenedcomparing to the conventional beam scanning method using a cylindricalconvex lens effect. Particularly in a current state, only theacoustooptical element made of quartz having a fast sonic speed(ultrasonic propagation speed) can be used in a laser wavelength shorterthan 300 nm, a remarkable effect is exerted. For example, in the casewhere the propagation speed V is 5960 (m/sec.), the wavelength of thelaser beam is 244×10⁻⁹ (m), the sweeping time of ultrasonic (output timeof a series of ultrasonic) T is 2×10⁻⁶ (sec.), and the band ofultrasonic f is 80×10⁻⁶ (Hz), the focal length f is 3.64 m according tothe foregoing expression 1. Therefore, when the acoustooptical elementis made to exert the convex lens effect to relay the convergence spotand scanning is performed conventionally, the optical path lengthens bya length corresponding to about 2×f, which is not realistic when thestability is considered. On the other hand, according to the presentembodiment, since the acoustooptical element is used so as to functionas the concave lens, the virtual image is relayed, and thus the opticalpath of a scanning optical system can be shortened. For this reason, thestable scanning can be performed even when the acoustooptical elementmade of quartz having a fast ultrasonic propagation speed is used.

Next, description will be made for a second embodiment of the presentinvention. In the first embodiment, the frequency is swept such that thewavelength of the ultrasonic pulse of the acoustooptical element 4becomes shorter from the forefront portion 26 to the aftermost portion25, but the sweeping direction of the frequency is reversed in thesecond embodiment. FIG. 4 is a schematic view showing a structure of anoptical reticle substrate inspection apparatus according to the secondembodiment of the present invention and its beam scanning method. Notethat the same reference numerals are added to the same constituentelements as the first embodiment shown in FIG. 2 and FIG. 3, and theirdetail description will be omitted.

In the second embodiment, the acoustooptical element 4 is arranged suchthat a side where the transducer 19 is attached, that is, a side whereultrasonic is input is made far from the group of cylindrical lenses 3.Other part of the structure is the same as that of the first embodiment.

In the beam scanning method for the second embodiment constituted inthis manner, the transducer 19 outputs a series of ultrasonic such thatthe wavelength lengthens in linear state as the passage of time. Aplurality of parallel lines between the aftermost portion 27 and theforefront portion 28 in FIG. 4 show that the wider the distance betweenthe lines the longer the wavelength. The distance between the forefrontportion 28 and the aftermost portion 27 of the series of ultrasonicequals the width of the laser beam 30 similarly to the first embodiment.When the laser beam 30 is made incident to the acoustooptical element 4,diffraction of the laser beam 30 occurs due to the ultrasonic pulse, inwhich the frequency is swept such that the wavelength lengthens inlinear state as described above. Thus, the concave lens effect occurs inthis embodiment as well. Then, the circular virtual image 14 is formedon a side where the laser beam 30 is made incident by the combinationalfunction with the group of cylindrical concave lenses 5. When the laserbeam has its optical path on the path 13, the circular virtual image 15is formed on a position off from the virtual image 14 in the scanningdirection.

Therefore, an effect that the stable scanning can be performed isobtained by the second embodiment similarly to the first embodiment byperforming the same scanning as the first embodiment.

What is claimed is:
 1. A beam scanning method of an optical reticle substrate inspection apparatus, comprising the steps of: scanning a laser beam output from a laser in a first scanning direction using a first acoustooptical element; generating a virtual image with a second acoustooptical element using the laser beam output from the first acoustooptical element; magnifying said laser beam output from the second acoustooptical element in a second direction; and imaging said virtual image on a reticle substrate being an object to be inspected.
 2. The beam scanning method of the optical reticle substrate inspection apparatus according to claim 1, wherein a wavelength of said laser is 300 nm or less.
 3. The beam scanning method of the optical reticle substrate inspection apparatus according to claim 1, wherein at least one of said acoustooptical elements is made of quartz.
 4. The beam scanning method of the optical reticle substrate inspection apparatus according to claim 2, wherein at least one of said acoustooptical elements is made of quartz. 